1. Field of the Invention
The present invention relates to optical devices (for example, optical displays for numeral or letter expression or X-Y matrix expression, and optical filters for controlling light transmittance or reflectance therethrough within a visible light range (wavelength: 400 to 700 nm)), and to an electrolytic solution to be used in those devices.
2. Description of the Related Art
Electrochromic materials (hereinafter referred to as EC materials) have heretofore been being used in voltage-driving-type displays and employed, for example, in digital timepieces and the like.
Electrochromic displays (hereinafter referred to as ECD) are of a non-light-emitting type for image expression through reflected or transmitted light, and have the advantages of giving a few fatigue feelings in long-time observation and requiring relatively low driving voltage and small electric power. For example, as in Japanese Patent Application Laid-Open (JP-A) No. Sho-59-24879, known are liquid-type ECD that comprise an EC material of organic molecules of a viologen derivative capable of reversibly producing a condition of color expression and extinction.
With the development of precision optical instruments, required are fine and power-saving light modulators capable of being substituted for conventional variable ND filters. For this, ECD such as those noted above will have to be investigated as to whether or not their peripheral technology may apply to such devices.
However, using EC materials such as viologen derivatives in ECD is problematic in point of the response speed and the shieldability necessary in practical use, and it is difficult to put ECD into practical use. Further, though the light modulators are required such that the light transmittance can be controlled within a visible light range (wavelength: 400 to 700 nm), the above-described EC materials were not satisfactory with respect to their characteristics.
In place of ECD, transmission-type or reflection-type light modulators to be driven through deposition/dissolution of metal salts are noted, and electrochemical light modulators to be driven through deposition/dissolution of silver are being developed.
The response speed and the light shieldability of such electrochemical light modulators could be on a intended level, but the transparent electrode (that is, working electrode) to be the substrate of those devices is easily deteriorated so that the life of the devices is short.
In particular, ITO (indium tin oxide) electrodes are often in overpotential condition in the cycle of deposition and dissolution of silver, and are thereby damaged.
The reversibility of metal salts in conventional electrochemical light modulators is lowered in cycle use and the electrolytic solution therein is often discolored. Therefore, conventional electrochemical light modulators could not exhibit sufficient light-modulating activity. That is, in driving the devices, if the system was kept in a peroxidized state, side-reaction products such as iodine were generated due to 2I.sup.-.fwdarw.I.sub.2 +2e.sup.-, whereby the inside of the effective diameter was often stained.